The present invention relates to alarm systems and, more particularly, to a method and system for providing an audible alarm to alert occupants of a building and/or authorities to the presence of an abnormal condition such as an excess concentration of potentially combustible gaseous hydrocarbons within a building or area.
It has become desirable and, in many instances, even mandatory to provide alarm systems that sense abnormal or undesirable conditions and alert the occupants of a building to the presence of the abnormal condition so that appropriate action may be taken. Numerous types of smoke and fire detectors, gas detectors, intrusion detectors and other such alarm systems are currently in use for such purposes. In a typical residence, for example, three or four such systems for detecting different conditions may be in use.
The typical alarm system utilizes a sensor to sense a condition such as smoke or gas concentration and an audible alarm such as a buzzer, horn or siren to provide an alerting sound when the sensed condition is abnormal. The type of sensor employed will, of course, depend upon the condition being sensed. For smoke alarm systems, various types of photoelectric and ionization type sensors are employed to determine when the concentration of smoke in the air exceeds some predetermined value. Intrusion detectors utilize a large number of different types of sensors while typical gas alarms, usually for indicating the presence of gaseous hydrocarbons, employ a sensor which is basically a bulk semi-conductor material composed mainly of tin dioxide which, when heated, exhibits a resistance drop related to the concentration of gaseous hydrocarbons in the vicinity of the sensor.
One aspect of this invention relates broadly to various alarm systems. Within a single residence or other occupied space, there may be several alarms for different conditions. For example, there may be a burglar alarm to detect intrusion, a smoke alarm to detect combustion and a gas alarm to detect gas leaks. The usual alarm system provides an audible alarm through the energization of a transducer such as a piezoelectric or electromagnetic "buzzer". While there may be some difference between the sounds produced by the various alarms, they are heard very infrequently and it may be difficult to readily ascertain which alarm is sounding when the alarm condition is detected. It is, of course, extremely important to take relatively fast action when one of the alarms sounds so any delay in ascertaining which alarm has been triggered could be extremely dangerous or even fatal.
It is accordingly one object of the present invention to provide a novel method and system for providing an alarm signal which permits the immediate recognition of the sensed alarm condition through the provision of a spoken message.
Another more specific aspect of the present invention relates to alarm systems for detecting the occurrence of a predetermined concentration of gaseous hydrocarbons in an occupied area and providing an alarm. The usual gas detection system employs a sensor that is composed mainly of tin dioxide and exhibits certain variable resistance characteristics when heated and exposed to gaseous hydrocarbons. Typical of such gas sensors are the TGS 109, TGS 812 and TGS 813 gas sensors available from Figaro Engineering, Inc., of Japan. The Figaro gas sensors have a variable resistance element and a heater connected to electrodes in such a manner that the variable resistance element (the bulk semi-conductor tin dioxide) is heated when the sensor is energized. The resistance between two of the electrodes connected to the resistance element varies in accordance with known characteristics and can be sensed as an indication of gaseous hydrocarbon concentration.
Specifically, the resistance characteristics of this type of gas sensor are such that the resistance value is very high when the semi-conductor material is unheated and is also at a relatively high, stable resistance value after the semi-conductor has been heated for a predetermined time period, usually from 1 to 3 minutes. The resistance value remains at this relatively stable value as long as the heater is energized (barring failure) and decreases from the relatively stable value as a function of the concentration of gaseous hydrocarbons in the vicinity of the resistance element. However, upon initial energization of the heating element, there is a substantial decrease in the resistance value between the time of initial energization of the heating element and the end of the initial 1 to 3 minute heating interval.
In known gas alarms, the value of the resistance element is sensed in order to provide an alarm when the concentration of gaseous hydrocarbons exceeds some predetermined value, usually about 10% of the lower explosive level of the most common gas expected to be encountered. It will be appreciated that if the resistance of the sensor is directly sensed to provide an alarm, an alarm will occur sometime within the first second of initial turn-on as well as when the undesirable level of gas concentration is reached since the resistance at the undesirable gas concentration level is higher than the low value reached during initial turn-on. This is undesirable and, in fact, is not permitted in accordance with U. L. specifications.
Various circuits have been devised to prevent an alarm condition from occurring except after the resistance element has been heated and has reached its relatively stable value. These circuits are extremely complex and therefore are costly and more likely to fail in normal use. Moreover, when fault circuitry is added to detect common malfunctions of such alarm systems, they become even more complex, costly and prone to failure.
It is accordingly another object of this invention to provide a method and circuit for producing a first audible alarm only when the concentration of gaseous hydrocarbon in a protected area reaches a predetermined level and a second audible alarm upon occurrence of common malfunctions in the circuit, wherein complexity is minimized resulting in decreased cost and size, and increased reliability.